The word “Control System” has two separate words. First one is “Control” and next is “System”. Let us see, what is a System?
Definition: A system is a collection of elements or components which act together as an entire unit to achieve certain objective.
For example: A Classroom has desks, benches, board and students etc those act together to complete the objective of teaching-learning process. Some of the other exampes of systems are bulb, kite, trafic signal etc.
Thus, a system has input and output assosiated with it.
Control means manage, command, direct or regulate.
A control system manages, commands, directs or regulates the behavior of system. It is a system, which provides the desired response by controlling the output.
Definition: A control system is defined as a system of devices that manages, commands, directs, or regulates the behavior of other devices or systems to achieve a desired result.
For example: Fan with regulator, Classroom with professor/teacher and Bulb with switch.
Basic Definitions Related to Control System
A simplified diagram of control system is as follows.
Thus, a control system must have
- An arrangement of subsystems or elements to achieve desired result.
- Input: The excitation or stimulus applied to a system from an external source in order to produce an output is called input signal.
- Output: The actual signal obtained from the system is called an output response.
- Control action: Control action is that quantity responsible for activating the system to produce the output.
By configurations that is how subsystems are connected between input and output, the control systems mainly classified into
- Open loop system
- Closed loop system
Open Loop Control System
An open loop system is one which does not automatically correct the change in output by adjusting the input signal.
Definition: A system in which control action is totally independent on the output of the system is called as open loop system.
For example: Lamp, Fan, Traffic signal and Automatic door opening.
Here, an input is applied to a controller and it produces an actuating signal or controlling signal. This signal is an input to a plant or process which is to be controlled. So, the plant produces an output, which is controlled by controlling signal.
1) Controller: The element of the system itself or external to the system which control the plant or process is called controller.
For example: ON/OFF switch of the bulb, Regulator of the fan
2) Plant: The portion of the system to be controlled is called plant. A plant may be a piece of equipment or just a set of machine parts.
For example: Thermostat, filament or heating element.
3) Process: Process is any operation to be controlled.
For example: Chemical processes, pneumatics processes.
Open Loop Example
Consider a fan with regulator as an exampple of open loop control system. Following figure shows the block diagram of fan as an open loop system.
This diagram is modified from the generalized block doagram of open loop system. The regulator is controller and fan is plant.
If we switch ON the fan, it starts moving at a speed depending on the position of the regulator. It continues to rotate at same speed even though surrounding temperature changes.
The only way to change the speed of fan is to adjust the regulator physically. Therefore, it is an open loop system because the control action does not depend on output of system.
In this system even if the room temperature changes, the fan will not be able to change it’s speed without human interference.
Advantages of Open Loop Control Systems
- They are simple in construction.
- They are relatively more stable.
- They are cheaper and have less maintenance.
Disadvantages of Open Loop Control Systems
Inaccurate and unreliable.
To maintain quality and accuracy, recalibration of the system is necessary from time to time.
Closed Loop Control System
A closed loop system is one which automatically corrects the change in output by adjusting the input signal.
Definition: A system in which control action is dependent on the output of the system is called as closed loop system.
For example: Automatic electric irons, Air conditioners, Voltage stabilizers and Classroom with professor
In closed loop system output changes if the given input if the external conditions change. There is continuous correction in output. Following figure shows the generalized block digram of a closed loop system.
A Closed-loop Control System, also known as a feedback control system is a control system which uses the concept of an open loop system as its forward path but has one or more feedback loops or paths between its output and its input.
The reference to “feedback”, simply means that some portion of the output is returned “back” or “fed back” to the input to form part of the systems excitation.
Closed-loop systems achieve and maintain the desired output condition by comparing it with the actual condition. It does this by generating an error signal which is the difference between the output and the reference input.
In other words, a “closed-loop system” is a fully automatic control system in which its control action is dependent on the output in some way.
Closed Loop example
Consider a Home heating system. The block diagram of home heating system is shown in the following figure.
If the system is ON. The actual temperature is measured by temperature sensor.
The actual temperature and desired temperature is compared and if these are different then it actuates the valve mechanism to change the temperature as per requirement.
Therefore, it is a closed-loop control system because control action is dependant on output and surrounding changes.
Advantages of Closed Loop Control System
- Accuracy is high.
- It senses environmental changes and correct the output accordingly.
- High bandwidth.
- Reduced effect of non-linearities.
Disadvantages of Closed Loop Control System
- Complicated design so they are costlier.
- System may become unstable.
Comparison of Open and Closed Loop Control Systems
Open Loop System
Closed Loop System
Feedback is present.
Sensitive to parameters changes.
Less sensitive to parameters changes.
May become unstable.
Examples: fan, automatic toaster, Traffic signal etc.
Examples: AC, Missile launcher system, Voltage stabilizers etc.
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